When samples are smaller than about 1 mm by 1 mm in size, they are usually imaged by a broad class of instruments called microscopes. This broad class includes scanning laser microscopes, which subclass is further subdivided into scanning stage and scanning beam microscopes. In scanning stage laser microscopes, the sample is moved in a raster scan under a stationary focused laser beam. Such microscopes have good spatial resolution, but are slow. A prior art infinity-corrected scanning beam laser microscope is shown in FIG. 1. In this microscope beam 101 from laser 100 passes through a spatial filter and beam expander, comprised of lens 102, pinhole 104 and lens 106, and two scanning mirrors 110 and 116 deflect the beam in a raster scan. Lenses 112 and 114 bring the beam back to the axis so that it hits the center of scanning mirror 116, and lenses 118 and 120 bring it back to the axis as the beam, now with scan added, enters the entrance pupil of microscope objective 122. The laser beam is focused to a spot on sample 124 by microscope objective 122. Focusing of the microscope is accomplished by moving focusing stage 126. Light reflected back from the sample is collected by the microscope objective, passes back through the scan system, and is partly reflected by beamsplitter 108 into the detection arm, which is comprised of lens 128, pinhole 130 and detector 132. Light returning from the focused spot on the sample is focused by lens 128 to pass through pinhole 130 and reaches detector 132. Light from other points in the sample hits the edges of pinhole 130 and is not detected. When pinhole 130 is used in this way, this is said to be a confocal microscope, and it has optical image slicing ability, which allows it to record true three-dimensional images. In this type of microscope the scanning beam is expanded by the beam expander to fill the entrance pupil of the microscope objective, and passes through the optical axis just as it enters the microscope objective. With a large numerical aperture (NA) 100.times.microscope objective, the scan size is about 300 by 300 microns, and with a low power objective, with lower NA, the scan size reaches 1 mm by 1 mm. Confocal scanning beam laser microscopes are also often used for fluorescence and photoluminescence imaging. A transmission and reflection scanning beam confocal microscope was described by Dixon.sup.1 et al and in U.S. Pat. No. 5,386,112. A second design for a transmission microscope is disclosed in DE,A,3918412. In this second design the maximum scan angle is severely limited by the large distance from the last scan lens to the microscope objective, resulting in a scan length that is even smaller than in an ordinary scanning beam microscope.
There is a broad class of instruments used to form images of macroscopic samples that are larger than the samples usually used in a microscope, that is, larger than about 1 mm.times.1 mm in size. These instruments use several different contrast mechanisms, including those listed in the "Technical Field" description above. The invention described in this application relates primarily to the imaging of macroscopic samples.
Photoluminescence scanning or mapping of semiconductor wafers is a valuable technique for quality control of wafers and epitaxial layers in the semiconductor industry. One method is to keep the wafer stationary and raster the beam using a gimbaled mirror, as described by Hovel.sup.2. This has the advantage of being inexpensive, but the laser spot does not stay in focus across the wafer, thus resulting in poor quality images. A second method is to use computer-controlled x-y tables to move the wafer under a stationary focused laser beam, as described by Hovel.sup.2 and by Moore and Miner.sup.3. This is essentially the use of a non-confocal scanning-stage laser microscope to measure PL across the wafer, and a monochromator is often used to enable PL to be measured as a function of wavelength. This method gives good spatial and spectral resolution, but is slow, since the scan speed is limited by the speed of the moving tables. The combination of a scanning-stage PL mapping system with an apparatus and method for measuring film thickness is described by Miner.sup.4. A third method of measuring PL maps of large samples is described by Carver.sup.5. He uses a scanning beam laser microscope to form high resolution images of semiconductor samples, covering an area of 250 microns by 250 microns, and then translates the sample to image other areas. A fourth prior art system is described by Steiner and Thewalt.sup.6. This system uses a cooled ccd array to image whole wafers, up to 100 mm in diameter, making rapid absorption or photoluminescence maps, and has been used to map EL2 concentration and donor-acceptor pair band photoluminescence intensity in semi-insulatinig GaAs wafers. Because the ccd array acquires the entire image simultaneously, this system is very fast. Wavelength selectivity is accomplished using interference filters. A complete photoluminescence spectrum of any particular spot on the wafer is acquired with a separate system, using a focused Ar ion laser beam and a remote Fourier transform interferometer. This system has several disadvantages. First, in the whole-wafer imaging mode, it is not possible to acquire complete spectra from each sample position. Second, the intensity of the exciting source at any sample position is severely limited since the whole wafer is illuminated, and the input power required to match the illumination intensity of a laser source illuminating only a single point would cause considerable heating of the wafer. Third, the resolution across the sample is limited to the number of pixels in the ccd camera. Fourth, the sensitivity of detection is limited to the sensitivity of a single detector element in the ccd array.
An apparatus and method for imaging defects in semiconductor wafers is the scanning infrared microscope (SIRM) described by Booker et al.sup.7. In this apparatus, a fixed detector placed behind a semiconductor sample detects light transmitted through the wafer from a fixed laser and lens combination. An image of the wafer in transmission is recorded by mechanically raster scanning the sample under the fixed beam. This system gives good resolution, but is slow.
The prior art imaging systems described herein and in the reference literature are used generally for reflected light and transmitted light imaging, as well as for PL and FL imaging, but several other contrast mechanisms are possible. These systems have several limitations. The scanning stage systems are slow, because they mechanically scan the sample under a fixed laser beam. Camera-based systems record the image much more rapidly, but the image resolution is limited by the number of detector elements in the ccd array, and it is difficult to get good spectrally-resolved data at each pixel position. Detector sensitivity is limited to the sensitivity of each detector element in the array. In addition, the maximum illumination intensity is limited by heating of the sample.